Topic spaces

A topic space is the set of topics on which you can subscribe and publish. A queue manager in a distributed publish/subscribe topology has a topic space that potentially includes topics that have been subscribed and published to on connected queue managers in that topology.

Note: For an overview of topics within a queue manager, such as administrative topic objects, topic strings and topic trees, seeTopics. Further references to topics in the current article refer to topic strings unless otherwise specified.
Topics are initially created in either of the following ways:
  • administratively, when you define a topic object or durable subscription.
  • dynamically, when an application creates a publication or subscription dynamically to a new topic.

Topics are propagated to other queue managers both through proxy subscriptions, and by creating administrative cluster topic objects. Proxy subscriptions result in publications being forwarded from the queue manager to which a publisher is connected, to the queue managers of subscribers.

Proxy subscriptions are propagated between all queue managers that are connected together by parent-child relationships in a queue manager hierarchy. The result is, you can subscribe on one queue manager to a topic defined on any other queue manager in the hierarchy. As long as there is a connected path between the queue managers, it does not matter how the queue managers are connected.

Proxy subscriptions are also propagated for subscriptions to cluster topics in a publish/subscribe cluster. A cluster topic is a topic that is attached to a topic object that has the CLUSTER attribute, or inherits the attribute from its parent. Topics that are not cluster topics are known as local topics and are not replicated to the cluster. No proxy subscriptions are propagated to the cluster from subscriptions to local topics.

To summarize, proxy subscriptions are created for subscribers in two circumstances.
  1. A queue manager is a member of a hierarchy, and a proxy subscription is forwarded to the parent and children of the queue manager.
  2. A queue manager is a member of a cluster, and the subscription topic string resolves to a topic that is associated with a cluster topic object. When the topic is a direct routed cluster topic, proxy subscriptions are forwarded to all members of the cluster. When the topic is a topic host routed cluster topic, proxy subscriptions are forwarded only to the queue managers in the cluster that have defined the clustered topic object. For more information seePublish/subscribe clusters.
If a queue manager is a member of a cluster and a hierarchy, proxy subscriptions are propagated by both mechanisms without delivering duplicate publications to the subscriber.
The topics spaces of three publish/subscribe topologies are described in the following list: In separate topics, the following configuration tasks describe how to combine topic spaces.

Case 1. Publish/subscribe clusters

In the example, assume that the queue manager is not connected to a publish/subscribe hierarchy.

If a queue manager is a member of a publish/subscribe cluster, its topic space is made up from local topics and cluster topics. Local topics are associated with topic objects without the CLUSTER attribute. If a queue manager has local topic object definitions, its topic space is different from another queue manager in the cluster that also has its own locally defined topic objects.

In a publish/subscribe cluster, you cannot subscribe to a topic defined on another queue manager, unless the topic you subscribe to resolves to a cluster topic object.

When the same named definitions of a cluster topic object are required on multiple queue managers, for example when using topic host routing, it is important that all definitions match where necessary. For more information, see Creating a single topic space in a publish/subscribe cluster.

A local definition of a topic object, whether the definition is for a cluster topic or a local topic, takes precedence over the same topic object defined elsewhere in the cluster. The locally defined topic is used, even if the object defined elsewhere is more recent.

It is important that a cluster topic object is associated with the same topic string everywhere in the cluster. You cannot modify the topic string with which a topic object is associated. To associate the same topic object with a different topic string, you must delete the topic object and re-create it with the new topic string. If the topic is clustered, the effect is to delete the copies of the topic object stored on the other members of the cluster and then to create copies of the new topic object everywhere in the cluster. The copies of the topic object all refer to the same topic string.

It is possible to accidentally create two definitions of the same named topic object on different queue managers in the cluster, with different topic strings. This can result in confusing behavior, because multiple definitions of the same topic object with different topic strings can produce different results depending how and where the topic is referenced. See Multiple cluster topic definitions of the same name for more information on this important point.

Case 2. Publish/subscribe hierarchies in version 7 or later versions

In the example, assume that the queue manager is not a member of a publish/subscribe cluster.

In version 7 or later, if a queue manager is a member of a publish/subscribe hierarchy, its topic space consists of all the topics defined locally and on connected queue managers. The topic space of all the queue managers in a hierarchy is the same. There is no division of topics into local topics and global topics.

Set either of the PUBSCOPE and SUBSCOPE options to QMGR, to prevent a publication on a topic flowing from a publisher to a subscriber connected to different queue managers in the hierarchy.

Suppose you define a topic object Alabama with the topic string USA/Alabama on queue manager QMA. The result is as follows:
  1. The topic space at QMA now includes the topic object Alabama and the topic string USA/Alabama.
  2. An application or administrator can create a subscription at QMA using the topic object name Alabama.
  3. An application can create a subscription to any topic, including USA/Alabama, at any queue manager in the hierarchy. If QMA has not been defined locally, the topic USA/Alabama resolves to the topic object SYSTEM.BASE.TOPIC.

Case 3. Publish/subscribe hierarchies and streams in Version 6

Before version 7, the topic space was divided into separate streams, which included the default stream that was present on all queue managers. Publications cannot flow between different streams. If named streams are used, the topic spaces at different queue managers might be different. Topics are divided into topics in the default stream, and topics in different named streams.
Note: Each named stream forms a separate topic space. To form a connected topology each named stream must exist on the connected queue managers. Suppose stream X is defined on QMA and QMC, but not on QMB. If QMA is the parent of QMB, and QMB is the parent of QMC, no topics in stream X can flow between QMA and QMC.

Setting both of the PUBSCOPE and SUBSCOPE options either to QMGR or to ALL requires a publisher and subscriber to a topic to exchange only publications for local consumption, or to exchange only publications for global consumption.

From version 7, streams are not available using the publish/subscribe API. If you use queued publish/subscribe on a version 7 queue manager, streams are mapped to different topic objects that can simulate the effect of streams. A stream is simulated by creating a topic object that is the root topic for all the topics in the stream. The queue manager maps publications and subscriptions between the stream and the corresponding root topic of each tree.